1. Field of the Invention
The present invention relates to a block copolymer composition comprising a block copolymer and a polystyrene resin. More particularly, the present invention is concerned with a block copolymer composition comprising a block copolymer (I) comprising at least two polymer blocks (each polymer block (S) comprising vinyl aromatic hydrocarbon monomer units) and at least one polymer block (B) comprising isoprene monomer units and 1,3-butadiene monomer units, and a polystyrene resin (II), wherein the amount of the vinyl aromatic hydrocarbon monomer units in the block copolymer (I), the total amount of the isoprene monomer units and the 1,3-butadiene monomer units in the block copolymer (I), and the isoprene monomer unit/1,3-butadiene monomer unit weight ratio in the block copolymer (I) are within specific ranges, wherein the block copolymer (I) has a vinyl aromatic hydrocarbon block ratio within a specific range, and wherein the block copolymer (I) and the polystyrene resin (II) satisfy a specific relationship.
A shaped article obtained from the block copolymer composition of the present invention exhibits not only less occurrence of fish eyes, which are gelled masses of a resin (hereinafter, the “fish eye” is frequently referred to simply as an “FE”), but also excellent impact resistance at a temperature as low as 0° C. or lower, excellent transparency, excellent stiffness and excellent elongation. Therefore, the block copolymer composition of the present invention can be advantageously used for producing various shaped articles.
2. Prior Art
A block copolymer comprising vinyl aromatic hydrocarbon monomer units (such as styrene monomer units) and conjugated diene monomer units (such as 1,3-butadiene monomer units) wherein the vinyl aromatic hydrocarbon monomer unit content is relatively high, has various excellent characteristics, such as high transparency and high impact resistance, so that the block copolymer has been used for producing injection-molded products, extrusion-molded products (such as a sheet and a film) and the like. Further, a composition comprising the block copolymer and a styrene polymer has excellent transparency and mechanical properties, so that the composition has been used for producing a sheet, a film and the like.
With respect to the above-mentioned block copolymers and compositions containing the same, a number of proposals have been made in patent documents. For example, in an attempt to improve the transparency and impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 52-58788 (corresponding to U.S. Pat. No. 4,167,545) discloses a branched block copolymer comprising a vinyl aromatic hydrocarbon and 1,3-butadiene, which is obtained by division-wise adding a catalyst to a block copolymerization system. Unexamined Japanese Patent Application Laid-Open Specification No. Sho 53-8688 (corresponding to U.S. Pat. No. 4,120,915) attempts to obtain a block copolymer having an improved thermal stability by a method comprising polymerizing a vinyl aromatic hydrocarbon to obtain a vinyl aromatic hydrocarbon polymer, followed by addition of 1,3-butadiene and a small amount of isoprene to perform a coupling reaction. In an attempt to improve the impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-113007 discloses a block copolymer comprising a vinyl aromatic hydrocarbon polymer block and a polymer block composed mainly of isoprene, which has a specific block configuration. As a thermoplastic polymer composition having excellent appearance (such as high transparency or high surface luster) and excellent impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 58-141233 discloses a composition comprising a vinyl aromatic hydrocarbon/1,3-butadiene block copolymer and a styrene polymer, wherein the block copolymer is a mixture of polymer chains which have largely different molecular weights and different compositions. For improving the environmental stress cracking resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-277509 (corresponding to U.S. Pat. No. 5,227,419 and European Patent No. 492490) proposes a method for producing a block copolymer which comprises a vinyl aromatic hydrocarbon and 1,3-butadiene and has a tapered block configuration, wherein the method comprises division-wise adding a catalyst to a block copolymerization system. For obtaining a multi-layer sheet made of a thermoplastic resin (a vinyl aromatic hydrocarbon/isoprene block copolymer) which has high stiffness and is improved with respect to rapid shaping at low temperatures and deep-draw shaping, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-177777 proposes a multi-layer sheet comprising a surface layer made of a first resin having a specific elastic modulus and a layer made of a second resin, wherein the Vicat softening point ratio of the first resin to the second resin is within a specific range. In an attempt to obtain a block copolymer having excellent transparency and mechanical properties, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-145314 (corresponding to U.S. Pat. No. 4,939,208 and European Patent No. 270515) discloses a method for producing a block copolymer having an S-B-B/S-S block configuration, wherein each S independently represents a styrene polymer block, B represents a butadiene polymer block, and B/S represents a butadiene/styrene copolymer block. Further, with a view to improving the transparency and impact resistance of a block copolymer and a composition containing the block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-97418 proposes a vinyl aromatic hydrocarbon/butadiene block copolymer and a composition comprising the block copolymer and a styrene polymer, in which the block copolymer has characteristic features with respect to the block ratio (the ratio of the total weight of vinyl aromatic hydrocarbon polymer blocks to the total weight of the vinyl aromatic hydrocarbon monomer units), the arrangement of the polymer blocks, the ratio of the butadiene in a vinyl aromatic hydrocarbon/butadiene random copolymer segment, and the like.
However, the conventional block copolymers comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, and the compositions comprising the block copolymers and a styrene polymer, which block copolymers and compositions are disclosed in the above-mentioned patent documents, have a problem in that shaped articles (such as a sheet) obtained from the block copolymers and the compositions containing the same suffer marked occurrence of FE's which are gelled masses of a resin. The occurrence of FE's in a shaped article is disadvantageous, for example, in that, when the shaped article is subjected to printing, the quality of the resultant print inevitably becomes poor.